1. Field of the Invention
This invention relates to optical finder devices suitable for still cameras, electronic cameras and video cameras.
2. Description of the Prior Art
Two types of optical finders have been known, one is the simple or external type used in rangefinders and leaf shutter type cameras, and the other is the through-the-lens (TTL) type used in the single lens reflex cameras, 8 mm cameras and cine cameras. Although the construction of the latter is more complicated, it is superior in that there is no parallax between the fields of view of the photographic lens and the finder. Nowadays, most high grade cameras employ the TTL finder, and in video cameras where the output signals from the image pickup tube may be routed to form an image to be observed, the electronic viewfinder has been widely used. In this case the advantage is that it can be monitored, but the disadvantages are that distance measurement accuracy is lower due to poorer resolving power, the bulk and size are greater, and electrical power consumption is more than with the optical finder. Therefore, it is difficult to decide which of the optical and electronic finders is superior. Generally speaking, however, in either case, it is desirable to increase the brightness and image magnification of the finder image.
The optical finders of the TTL type may be classified into two types, one which has only one image plane as used in the single lens reflex camera and the other which has two image planes as used in the 8 mm camera or the cine camera.
FIG. 1 schematically illustrates a finder optical system of the single lens reflex camera which is a prior art example of the former or primary imaging type finder. There are shown a photographic lens 1, a quick return mirror 2, a film 3, a focusing screen 4, a condenser lens 5, a pentagonal roof type prism 6, an eyepiece 7 and an exit pupil 8. Light entering through the photographic lens 1 is reflected by the quick return mirror 2 to the focusing screen 4 in a position equivalent to that of the film plane 3 on which a first image is formed. Light from this image is reflected by a roof face "a" and a face "b" of the pentagonal roof type prism 6 to the eyepiece 7 through which the photographer observes that image. The condenser lens 5 is necessary for pupil adjustment.
In the finder of FIG. 1, image magnification depends on the focal length of the eyepiece 7, which is subject to restriction by the air reduced length of the optical path through the pentagonal roof type prism 6, as expanded. To increase image magnification, it is necessary only to shorten the focal length of the eyepiece 7. Actually, however, because the size of the prism 6 can not be reduced from the required value for the roof reflection face "a" to admit of all light from the focusing screen 4, it is difficult to achieve the desired decrease in the eyepiece 7 focal length. Furthermore, even when the focal length of the eyepiece 7 can be shortened, another problem is that the exit pupil 8 position cannot be taken at a sufficiently long distance.
FIG. 2 in a schematic view illustrates the prior art example of the secondary imaging type of optical finder used in the 8 mm camera or cine camera, wherein there are shown the photographic lens 1 with a zoom system 1' and a relay system 1", the film plane 3, a half prism 9, an objective lens 10, a first image plane 11, a total reflection mirror 12, an erector lens 13, a second image plane 14 and an eyepiece 15. An afocal light beam emerging from the zoom system 1' is partly deflected upward by the half prism 9 and then focused by the objective lens 10 to form an image on the first plane 11. This image is reflected by the mirror 12 and then re-focused by the erector lens 13 to form a second image on the plane 14, which is observed through the eyepiece 15. Meanwhile, the photographic light passes through the relay system 1" to form an image on the film plane 3.
The secondary imaging type finder, in comparison with the primary imaging type finder of FIG. 1, has an additional lens system (in the instance of FIG. 2, lens 13) between the first and second image planes 11 and 14, and, moreover, is able to shorten the focal length of the eyepiece without being limited by the pentagonal roof type prism 6. Therefore, angular magnification can be increased. But the complexity of the structure increases, the length of the body in the longitudinal direction increases and the space the finder occupies increases. For these reasons, this type finder is not suitable for use in the single lens reflex camera, which must be minimal in bulk and size.
An object of the present invention is to provide a primary imaging type TTL viewfinder.
Another or second object is to make it possible to shorten the focal length of the eyepiece to heighten image magnification.
A third object is to make the structure of the viewfinder simple and small.